With the advent of small, low power stand-alone electronic type cameras for the detection of visible, near-infrared (VNIR) and long-wave infrared (LWIR) images (such as the conventional image-intensified CMOS imager for VNIR image detection and the uncooled micro-bolometer focal plane array imager for LWIR image detection), the number of applications for these types of VNIR or LWIR cameras has expanded to include hand-held, weapon-borne and helmet-mounted applications. However, the conventional VNIR and LWIR cameras each have an image-forming optical systems or objective lens assembly that is typically too long in a field-of-view axis to be suitable for use in hand-held, weapon-borne and helmet-mounted applications.
For example, FIGS. 1 and 2 depict a front and side view, respectively, of a conventional LWIR camera 50 and a conventional VNIR camera 60 each mounted to a helmet 70 having a body axis coordinate system 72. The coordinate system 72 includes a z-axis 74, a y-axis 76, and a x-axis 78. As shown in FIGS. 1 and 2, the z-axis 74 corresponds to a longitudinal axis of the helmet 70 running from the front to the back of the helmet 70. The y-axis 76 is orthogonal to the z-axis 74 and runs from the bottom to the top of the helmet 70. The x-axis 78 is orthogonal to the z-axis 74 and the y-axis 76.
The LWIR camera 50 and the VNIR camera 60 each include an objective lens assembly 52 and 62, respectively, and a corresponding image detector and processor 54 and 64, respectively, for detecting and processing an image received via the objective lens assembly 52 or 62. The LWIR and VNIR cameras 50 and 60 are shown in ajuxtaposed relationship, and arranged in such a way that their respective axial principal rays 58 and 68 are received along and define an optical path parallel to the z-axis 74 of the helmet 70. The optical path of each of the cameras 50 and 60 is also displaced from one another along the y-axis 76 of the helmet 70. In the conventional system depicted in FIGS. 1 and 2, the optical path of the LWIR and VNIR cameras 50 and 60 are positioned in close proximity along the y-axis 76 of the helmet in order to minimize sensor-to-sensor (e.g., camera 50 to camera 60) parallax.
Each objective lens assembly 52 and 62 is constructed as a series of lens elements or lens groupings (not shown in the figures), sequentially arranged and separated along their respective axial principal ray or optical path 58 and 68 and which define a respective front vertex-to-image (or entrance pupil to exit pupil) distance 58 and 68 for the respective conventional objective lens assembly 52 and 62. The telephoto ratio of each objective lens assembly 52 and 62 is approximately 1.7/1. As shown in FIG. 2, despite the relatively short front vertex-to-image distances 56 and 66, each of the conventional objective lens assemblies 52 and 62, when attached to their respective image detector and processor 54 or 64 and mounted on the helmet 70, exhibit an excessively long forward projection in the z-axis 74 of the helmet 70. The forward projection of each of these conventional objective lens assemblies 52 and 54 adversely affects the individual movement of the user wearing the helmet 70, interfering with other equipment or objects near the user. For example, this forward projection problem can inhibit a soldier's choice of weapon firing positions.
In general, for an objective lens assembly to be useful in the near infrared and far infrared wavebands, the respective relative aperture of the objective lens assembly must be small, such as, for example, in the approximate range of f/1.0 to f/1.2. Further, to be useful in a helmet-mounted application, the angular field-of-view must be comparatively large, such as, for example, 30° in elevation (vertical) and 40° in azimuth (horizontal).
Conventional objective lens assemblies having an effective focal length that is longer than the overall length of the respective prior art objective lens assembly are well known. When the ratio of overall length to focal length is less than unity, an objective lens assembly is referred to as telephoto. Unfortunately, telephoto lenses are not suited for applications requiring small relative apertures and large fields of view because they are prone to sizeable residuals of a backward curving field and both axial and lateral chromatic aberration.
Certain conventional objective lens assemblies have been designed to meet the aperture and field requirements mentioned above. However, the ratio of overall length to focal length (telephoto ratio) of each of these objective lens assemblies generally is greater than 1.7. When such an objective lens assembly is combined with an electronic camera, the overall optical path inclusive of the objective lens and camera body is too long (in the direction perpendicular to the camera focal plane array) for the helmet-mounted application, causing the forward projection problem discussed above.
One conventional objective lens assembly or optical system employed in small-sized or hand held cameras uses two prisms to fold an optical path as disclosed in Nagata, U.S. Pat. No. 6,900,950. The first prism has a first transmitting surface, a first reflecting surface and a second transmitting surface. The second prism has a first transmitting surface, a first reflecting surface, a second reflecting surface and a second transmitting surface. In that arrangement, the optical path propagation through the second prism, as a result of two reflections, has a perpendicular intersection, whereafter the emerging light path is parallel to and displaced transversely from the initial light path from the source, which is at normal incident on the first prism face. However, this conventional optical system with electronic camera attached is neither satisfactorily thin nor compact. Moreover, this conventional optical system makes use of asymmetric free-form aspheric surfaces that require expensive tooling, which may require a large production base to satisfactorily amortize the tooling cost.
A second conventional objective lens assembly or optical system that uses two prisms is disclosed in Nagata, U.S. Pat. No. 6,876,390. The first and second prisms of this second conventional optical system each has a first surface through which a light beam enters the prism, a second surface reflecting the incident light beam in the prism, a third surface reflecting the reflected light beam in the prism, and a fourth surface through which the light beam exits the prism. The large number of reflecting surfaces in this conventional optical system has the disadvantage of being extremely sensitive to tilt and decenter errors. Since at least two reflecting surfaces in each prism are rotationally asymmetric free-form aspheric surfaces, fabricating each of the prisms in this second conventional optical system is costly. Finally, this second conventional optical system alone or when attached to an electronic camera is neither thin nor compact for a helmet-mounted application.
A third conventional objective lens assembly or optical system that uses two prisms is disclosed in Aoki et al., U.S. Pat. No. 6,084,715. The first and second prisms of this second conventional optical system are separated transversely with an aperture stop disposed between the two prisms. The first prism has an object facing reflecting surface with an aspheric surface attribute that reflects the light beam in the prism, providing power to the light beam as it is reflected. The second prism has an image facing reflecting surface with an aspheric surface geometry that reflects the light beam in said prism, also giving power to the light beam upon reflection. The entrance and exit faces of each prism have aspheric surface attributes. All of said aspheric surfaces are asymmetrical free-form surface geometries. Further, the large number of asymmetric free-form aspheric surfaces in this example has the disadvantage of being extremely sensitive to manufacturing errors. Moreover, this configuration when attached to an electronic camera is neither sufficiently thin nor compact for a helmet-mounted application.
Therefore, a need exists for an objective lens assembly that overcomes the problems noted above and others previously experienced for a VNIR camera or a LWIR camera suitable for use in hand-held, weapon-borne and helmet-mounted applications.